linux进阶19——GDB（五）：单步调试命令（next、step和until）_gdb until

一段c语言程序:

#include <stdio.h>
int print(int num){
    int ret = num * num;
    return ret;
}
int myfunc(int num){
    int i = 1;
    int sum = 0;
    while(i <= num){
        sum += print(i);
        i++;
    }
    return sum;
}
int main(){
    int num =0;
    scanf("%d", &num);
    int result = myfunc(num);
    printf("%d", result);
    return 0;
}

1. next命令

1.1 功能

next 是最常用来进行单步调试的命令，其最大的特点是当遇到包含调用函数的语句时，无论函数内部包含多少行代码，next 指令都会一步执行完。也就是说，对于调用的函数来说，next 命令只会将其视作一行代码。

1.2 语法

(gdb) next count

参数 count 表示单步执行多少行代码，默认为 1 行。

1.3 示例

[root@localhost day5]# gcc test1.c -o test1 -g
[root@localhost day5]# ls
test1  test1.c
[root@localhost day5]# gdb test1 -q
Reading symbols from /home/gdb/day5/test1...done.
(gdb) l 0
1	#include <stdio.h>
2	int print(int num){
3	    int ret = num * num;
4	    return ret;
5	}
6	int myfunc(int num){
7	    int i = 1;
8	    int sum = 0;
9	    while(i <= num){
10	        sum += print(i);
(gdb) 
11	        i++;
12	    }
13	    return sum;
14	}
15	int main(){
16	    int num =0;
17	    scanf("%d", &num);
18	    int result = myfunc(num);
19	    printf("%d", result);
20	    return 0;
(gdb) 
21	}
(gdb) 
Line number 22 out of range; test1.c has 21 lines.
(gdb) b 16
Breakpoint 1 at 0x4005c8: file test1.c, line 16.
(gdb) r
Starting program: /home/gdb/day5/test1 
 
Breakpoint 1, main () at test1.c:16
16	    int num =0;
Missing separate debuginfos, use: debuginfo-install glibc-2.17-323.el7_9.x86_64
(gdb) n 2
3
18	    int result = myfunc(num);
(gdb) n    <---------跳过myfunc函数，直接到19行的代码
19	    printf("%d", result);
(gdb) n
20	    return 0;
(gdb) p result
$1 = 14
(gdb) 

2. step命令

2.1 功能

通常情况下，step 命令和 next 命令的功能相同，都是单步执行程序。不同之处在于，当 step 命令所执行的代码行中包含函数时，会进入该函数内部，并在函数第一行代码处停止执行。

2.2 语法

(gdb) step count

2.3 示例

[root@localhost day5]# clear
[root@localhost day5]# gcc test1.c -o test1 -g
[root@localhost day5]# gdb test1 -q
Reading symbols from /home/gdb/day5/test1...done.
(gdb) l 0
1	#include <stdio.h>
2	int print(int num){
3	    int ret = num * num;
4	    return ret;
5	}
6	int myfunc(int num){
7	    int i = 1;
8	    int sum = 0;
9	    while(i <= num){
10	        sum += print(i);
(gdb) 
11	        i++;
12	    }
13	    return sum;
14	}
15	int main(){
16	    int num =0;
17	    scanf("%d", &num);
18	    int result = myfunc(num);
19	    printf("%d", result);
20	    return 0;
(gdb) 
21	}
(gdb) b 18
Breakpoint 1 at 0x4005e5: file test1.c, line 18.
(gdb) r
Starting program: /home/gdb/day5/test1 
3
 
Breakpoint 1, main () at test1.c:18
18	    int result = myfunc(num);
Missing separate debuginfos, use: debuginfo-install glibc-2.17-323.el7_9.x86_64
(gdb) step     <------------程序进入mufunc函数，并在第一行代码处停止
myfunc (num=3) at test1.c:7
7	    int i = 1;
(gdb) 

3. until命令

3.1 语法

1、(gdb) until
2、(gdb) until location

其中，参数 location 为某一行代码的行号。until 命令可以简写为 u 命令。

不带参数的 until 命令，可以使 GDB 调试器快速运行完当前的循环体，并运行至循环体外停止。注意，until 命令并非任何情况下都会发挥这个作用，只有当执行至循环体尾部（最后一行代码）时，until 命令才会发生此作用；反之，until 命令和 next 命令的功能一样，只是单步执行程序。

3.2 示例

[root@localhost day5]# gdb test1 -q
Reading symbols from /home/gdb/day5/test1...done.
(gdb) l 0
1	#include <stdio.h>
2	int print(int num){
3	    int ret = num * num;
4	    return ret;
5	}
6	int myfunc(int num){
7	    int i = 1;
8	    int sum = 0;
9	    while(i <= num){
10	        sum += print(i);
(gdb) 
11	        i++;
12	    }
13	    return sum;
14	}
15	int main(){
16	    int num =0;
17	    scanf("%d", &num);
18	    int result = myfunc(num);
19	    printf("%d", result);
20	    return 0;
(gdb) 
21	}
(gdb) 
Line number 22 out of range; test1.c has 21 lines.
(gdb) b 17
Breakpoint 1 at 0x4005cf: file test1.c, line 17.
(gdb) r
Starting program: /home/gdb/day5/test1 
 
Breakpoint 1, main () at test1.c:17
17	    scanf("%d", &num);
Missing separate debuginfos, use: debuginfo-install glibc-2.17-323.el7_9.x86_64
(gdb) u
3
18	    int result = myfunc(num);
(gdb) step
myfunc (num=3) at test1.c:7
7	    int i = 1;
(gdb) u
8	    int sum = 0;
(gdb) u
9	    while(i <= num){
(gdb) u
10	        sum += print(i);
(gdb) u
11	        i++;
(gdb) u
9	    while(i <= num){
(gdb) u    <-------------重点:快速执行完循环体
13	    return sum;
(gdb) p sum
$1 = 14
(gdb) 

until 命令还可以后跟某行代码的行号，以指示 GDB 调试器直接执行至指定位置后停止。举个例子：

[root@localhost day5]# gdb test1 -q
Reading symbols from /home/gdb/day5/test1...done.
(gdb) l 0
1	#include <stdio.h>
2	int print(int num){
3	    int ret = num * num;
4	    return ret;
5	}
6	int myfunc(int num){
7	    int i = 1;
8	    int sum = 0;
9	    while(i <= num){
10	        sum += print(i);
(gdb) 
11	        i++;
12	    }
13	    return sum;
14	}
15	int main(){
16	    int num =0;
17	    scanf("%d", &num);
18	    int result = myfunc(num);
19	    printf("%d", result);
20	    return 0;
(gdb) 
21	}
(gdb) 
Line number 22 out of range; test1.c has 21 lines.
(gdb) b 17
Breakpoint 1 at 0x4005cf: file test1.c, line 17.
(gdb) r
Starting program: /home/gdb/day5/test1 
 
Breakpoint 1, main () at test1.c:17
17	    scanf("%d", &num);
Missing separate debuginfos, use: debuginfo-install glibc-2.17-323.el7_9.x86_64
(gdb) until 19     <-------------------直接执行至第19行停止
3
main () at test1.c:19
19	    printf("%d", result);
(gdb) p result   <---------------打印result结果
$1 = 14
(gdb) 

可以看到，通过执行 until 19 命令，GDB 调试器直接从第 17 行代码处执行至指定的第 19 行。